linux-kernel - Re: [RFC PATCH v2] random: add get_random

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Message-Id: <201903281000.x2SA0RcA017002@sdf.org>
Date:   Thu, 28 Mar 2019 10:00:27 GMT
From:   George Spelvin <lkml@....org>
To:     linux-kernel@...r.kernel.org, lkml@....org, tytso@....edu
Cc:     Jason@...c4.com
Subject: Re: [RFC PATCH v2] random: add get_random_max() function

By the way, I just noticed that my fallback get_random_max64()
algorithm (if there's no __int128 type) is completely broken and
will need rewriting.

It would work if I rejected and regenerated the high half
if the low half were out of range, but that's not what it does.

The worst case is a range of 0x10000001, where it would return
0x10000000 half the time.

Needs rethinking to find something as simple as possible.
I'm sure I can come up with something, but I'm not averse to
suggestions if anyone has any.

(If I had a reliably fast clz/fls, that would open some
possibilities, but sigh...)